Sealed secondary batteries that can be charged and discharged have high energy density. When an internal or external short circuit occurs, or the battery experiences abnormal heat generation or impact, charge/discharge reaction or chemical reaction occurs rapidly in the battery to cause rapid gas generation. This may expand or break a battery case. For this reason, most of the sealed secondary batteries are provided with a safety valve (an explosion-proof mechanism) through which the gas generated in the battery is released to outside the battery when the internal pressure of the battery reaches a predetermined value.
In contrast, known sealed secondary batteries with enhanced energy density each include an electrode group formed by winding or stacking a positive electrode plate and a negative electrode plate with a separator interposed therebetween, and a battery case in which the electrode group and an electrolyte are housed. Here, a sealing plate includes a safety valve, and seals an opening of the battery case with a gasket interposed therebetween. The positive electrode plate is connected through a positive electrode lead to the sealing plate, and the negative electrode plate is connected through a negative electrode lead to the inside bottom of the battery case. In this case, insulating plates are placed above and under the electrode group. These insulating plates prevent the electrode group from moving or being deformed in the battery case, and prevent the positive or negative electrode plate from being in contact with the negative or positive electrode lead, respectively, or the battery case to prevent an internal short-circuit.
Incidentally, when, in a sealed secondary battery with enhanced energy density, an electrode group is deformed due to an abrupt increase in internal temperature and pressure of the battery under abnormal conditions such as an internal short circuit, and thus blocks a safety valve, a battery case may rupture.
To address this problem, PATENT DOCUMENT 1 proposes a structure in which an insulating plate placed above the electrode group is made of phenol resin including glass cloth as a base. This insulating plate has high thermal resistance and high strength, and thus, prevents the deformation of the electrode group.